With the recent downsizing in the field of electronic devices including MEMS, an inkjet recording head and so on, there has been a growing demand for photosensitive resin compositions that allow fabrication of minute molded-resin products and formation of resin patterns with fine pattern widths of a few tens to a few hundreds of micrometers and high aspect ratios.
However, in the case of performing pattern formation by use of a photosensitive resin layer incorporating a traditional photosensitive resin composition which contained a novolak resin and diazonaphthoquinone as a photoacid generator, it was difficult to provide a profile having a high aspect ratio up to such a great height as a few hundred micrometers of film thickness. This was because, since the diazonaphthoquinone-type photoacid generator had high absorbency in the near ultraviolet region used for exposure, there was a great difference in exposure intensity of radiation between the top portion and the bottom portion of film when the photosensitive resin layer was thick film; as a result, the profiles of resin patterns formed had a taper shape or became contorted.
By contrast, there are proposals of photosensitive resin compositions capable of ensuring pattern profiles with high aspect ratios by containing combinations of photosensitive resins having functional groups, such as epoxy groups, with acid generators.
As to such photosensitive resin compositions, there are reports of, e.g., the light-curable composition which includes an epoxy-functional novolak resin, a cation-producing photoinitiator, such as a triarylsulfonium salt, and a diluent capable of reacting with an epoxy functional group and resists delamination because it is cured to perfection (Patent Document 1), the light-curable composition which includes a polyfunctional bisphenol A-formaldehyde novolak resin, triphenylsulfonium hexafluoroantimonate as an acid generator and a cyclopentanone solvent and allows formation of thick film (Patent Document 2), and so on.
On the other hand, a variety of acid generators other than diazonaphthoquinone-type acid generators have been developed for the purpose of providing highly sensitive resin compositions for optical molding. As examples thereof, aromatic sulfonium cationic polymerization initiators can be given. As to specific examples of photosensitive resin compositions containing such acid generators as an improvement factor, there are proposals of the resin compositions containing cation-polymerizable organic compounds and energy radiation-responsive cationic polymerization initiators (Patent Document 3 and Patent Document 4), and so on. In addition, the permanent resist composition containing a cationic polymerization initiator invested with an ability to absorb exposure light with wavelengths of 360 nm or above (Patent Document 5), and so on have been proposed too.    Patent Document 1: JP-B-7-78628    Patent Document 2: U.S. Pat. No. 6,391,523    Patent Document 3: JP-A-9-268205    Patent Document 4: JP-A-2005-055865    Patent Document 5: JP-A-10-097068